The present invention relates to a method of exploration of a medium by transmitting energy, and more particularly, a method of geophysical marine prospecting by creating seismic waves making it possible to obtain several distinct seismic profiles simultaneously of a selected geological region.
A method is known in which an energy pulse is transmitted from the transmission point, said energy pulse giving rise to mechanical waves in the medium to be explored which are capable of undergoing reflections particularly on the reflecting surfaces comprising the interfaces between successive geological layers of the said medium. This method, which is used with several sources of transmission either aligned or not with the receiver or receivers, is characterized in that the time intervals which separate two wave transmissions or consecutive pulses, each produced by one of the said transmission sources, are shorter than the time which it takes the longest wave to cover the return distance when being propagated in the medium to be explored, that each transmission source transmits waves separated by a time interval at least equal to the said time it takes the waves to cover the distance when being propagated in the medium, and that the points in time when transmission takes place are selected in such a way that, by cross-correlating the series of transmission times of at least one of the said transmission sources with the series of transmission times of all the sources, a function is obtained, during the course of the time defined by the said time which it takes the longest wave to cover the return distance when being propagated in the medium to be explored, in which the relationship between the peak maximum amplitude and the amplitude of each secondary residue is greater than a predetermined value, e.g. the relationship between the amplitudes of the signals received in the intervals of time corresponding to the intervals of time separating the maximum peak from each of the secondary residues or lobes.
This method makes it possible to use several transmission sources which are difficult or impossible to synchronize and to obtain information, without additional recording time, coming from the different reflection points; moreover, it also makes it easy to separate the information coming from the different reflection points and recorded simultaneously and thus to determine the detected reflecting surfaces precisely.
Thus when processing the recordings obtained by means of the receivers, separation of the information is carried out at the level of each transmission source, i.e. the proper information is determined at each transmission source. As the transmission sources are perfectly arranged in relation to the different receivers or geophones and especially as all of the transmitter-receiver sources are immobile during the course of the entire transmission it is possible, when there is seismic movement on land, to carry out the necessary number of transmissions in order that the crosscorrelation functions defined above by the transmission code should be satisfactory.
With marine seismology the transmission sources and receivers are displaced continuously and substantially at a constant speed; consequently for a given firing area which is carried and located there is the fear of not having enough information relating to a reflection point, which is defined by geophysicists as being the point in the strata of interface which is located on the mediator of the segment connecting a transmission source to the receiver with which it is associated.